A high-power ultrashort pulse semiconductor laser element made of a GaN-based compound semiconductor and having a light emitting wavelength in the 405 nm band holds promise as a light source of a volumetric optical disk system, which has potential as an optical disk system in the next generation to the Blu-ray (registered trademark) optical disk system, and as a light source required in medical fields, bioimaging fields, etc. As the method for producing short pulse light in a semiconductor laser element, mainly two methods of gain-switching and mode-locking are known; and the mode-locking is further categorized into active mode-locking and passive mode-locking. To produce light pulses on the basis of active mode-locking, it is necessary to configure an external resonator using mirrors or lenses and perform radio-frequency (RF) modulation on the semiconductor laser element. On the other hand, in passive mode-locking, by utilizing the self-pulsation operation of the semiconductor laser element, light pulses can be produced by simple DC driving.
To cause the semiconductor laser element to perform self-pulsation operation, it is necessary to provide a light emitting region and a saturable absorption region in the semiconductor laser element. Here, according to the arrangement state of the light emitting region and the saturable absorption region, the semiconductor laser element can be categorized into a saturable absorber layer (SAL) type or a weakly index guide (WI) type in which the light emitting region and the saturable absorption region are arranged in the vertical direction and a bi-section type in which the light emitting region and the saturable absorption region are juxtaposed in the resonator direction. The bi-section type semiconductor laser element is known from JP 2004-007002A and JP 2008-047692A, for example. A bi-section type GaN-based semiconductor laser element is considered to have a larger saturable absorption effect and be capable of producing a light pulse with a narrower width than the SAL type semiconductor laser element.
The bi-section type semiconductor laser element includes, for example, as shown in FIG. 18, which is a schematic cross-sectional view along the direction orthogonal to the direction in which the resonator extends (a schematic cross-sectional view taken along the YZ plane, i.e., a schematic cross-sectional view along arrows I-I in FIG. 19 and FIG. 20), in FIG. 19, which is a schematic end view along the direction in which the resonator extends (a schematic end view taken along the XZ plane, i.e., a schematic end view along arrows II-II in FIG. 18 and FIG. 20), and in FIG. 20, which is a schematic diagram of the semiconductor laser element as viewed from above,    (a) a stacked structure body in which a first compound semiconductor layer 30 having a first conductivity type and made of a compound semiconductor, a third compound semiconductor layer 40 that forms a light emitting region 41 and a saturable absorption region 42 made of a compound semiconductor, and a second compound semiconductor layer 50 having a second conductivity type different from the first conductivity type and made of a compound semiconductor are sequentially stacked,    (b) a second electrode 62, and    (c) a first electrode 61 electrically connected to the first compound semiconductor layer 30. The bi-section type semiconductor laser element has what is called a double-ridge structure in which a ridge stripe structure 71 formed of at least part of the stacked structure body is formed and a side structure body 72 (72A and 72B) formed of the stacked structure body is formed on both sides of the ridge stripe structure 71. The second electrode 62 formed on the second compound semiconductor layer 50 that forms the ridge stripe structure 71 is separated by an isolation trench 62C into a first portion 62A for sending a direct current to the first electrode 61 via the light emitting region 41 to create a forward bias state and a second portion 62B for applying an electric field to the saturable absorption region 42.
An insulating film 56 made of an oxide insulating material, such as SiOx, is formed to extend from on a portion of the ridge stripe structure 71 to on a portion of the side structure body 72, on which portions the second electrode 62 is not formed. The first portion 62A of the second electrode 62 is covered by a first lead-out wiring layer 63 provided on the insulating film 56 that is formed to extend from on the portion of the ridge stripe structure 71 to on the portion of the side structure body 72, and the second portion 62B of the second electrode 62 is covered by a second lead-out wiring layer 64 provided on the insulating film 56 that is formed to extend from the other side structure body 72B, over the ridge stripe structure 71, to the one side structure body 72A. A wire bonding layer 65 extending from the second lead-out wiring layer 64 is provided on the insulating film 56 formed on the one side structure body 72A. In FIG. 20, the first lead-out wiring layer 63, the second lead-out wiring layer 64, and the wire bonding layer 65 are shaded in order to show them clearly, and the first portion 62A and the second portion 62B of the second electrode are shown as well. In the first lead-out wiring layer 63 and the wire bonding layer 65, a gold wire, for example, is wire-bonded to the area enclosed by the dotted circle. Here, the reference numeral 73 represents a recess provided between the ridge stripe structure 71 and the side structure body 72, and the reference numeral 21 represents a substrate (specifically, an n-type GaN substrate, for example).